a(n) = 1/2 + (1-6*n)*(-1)^n/2.

Terms

a(0) =1a(1) =3a(2) =-5a(3) =9a(4) =-11a(5) =15a(6) =-17a(7) =21a(8) =-23a(9) =27a(10) =-29a(11) =33a(12) =-35a(13) =39a(14) =-41a(15) =45a(16) =-47a(17) =51a(18) =-53a(19) =57a(20) =-59a(21) =63a(22) =-65a(23) =69a(24) =-71a(25) =75a(26) =-77a(27) =81a(28) =-83a(29) =87